1. Field of the Invention
The invention relates to a magnetic reading device and, notably, to a reading head for magnetic recording media. It can be applied preferably in a magnetic reader of computer peripherals or in professional video tape recorders.
More particularly, the invention can be applied to a Kerr effect type of magneto-optical reading head with a view to linearizing its operation.
In the general context of a system for the recording and reading of data on magnetic media, the reading sensor may be magneto-optical and may use the "Kerr" effect to read the data elements.
The magneto-optical Kerr effect is a property of certain materials used in the industry of magnetic sensors: a linearly polarized optical beam that gets reflected on the surface of a material possessing the Kerr effect has its direction of polarization rotated as a function of the magnetic flux present in the material, at the position of the reflection.
As shown in FIGS. 1a and 1b, a reading sensor working by Kerr effect is organized in two layers of magnetic material 1, 2 separated by a non-magnetic insulator 3: this structure is optimized to channel the magnetic flux coming from the information elements recorded on the medium as is shown in FIG. 1a.
In the zone 5 of the sensor, this sensor is in contact or almost in contact with a magnetic medium 4 to be read. A linearly polarized optical beam FI gets reflected on one of the magnetic layers of the head and the information elements are read by detection of the rotation of polarization (the Kerr effect) of the reflected beam FR.
This device is the object of the French patent application No. 89 17313. However, this device has the following drawbacks:
The Kerr effect is a surface effect, and to obtain high sensitivity of the Kerr sensor, the layers forming the magnetic circuit have to be as thin as possible;
In structures such as these, the magnetization is organized in "magnetic domains" that shift randomly as a function of the magnetic information elements read back from the magnetic medium. The noise due to these random reconfigurations reduces the signal-to-noise ratio and makes the signal unusable;
Furthermore, the magnetic layers always display magnetic anisotropy in the plane and a residual coercive field that takes the form of non-linearities in the signal that is read back.
One technique used to stabilize the domains of the magnetic layers in magneto-resistive sensors consists of the addition, to the head, of a magnetized layer which "anchors" the domains by its field.
This technique is aimed at overcoming these defects.